Abstract/Summary

Climate models provide compelling evidence that if greenhouse gas emissions continue at
present rates, then key global temperature thresholds (such as the European Union limit of two
degrees of warming since pre-industrial times) are very likely to be crossed in the next few
decades. However, there is relatively little attention paid to whether, should a dangerous
temperature level be exceeded, it is feasible for the global temperature to then return to safer
levels in a usefully short time. We focus on the timescales needed to reduce atmospheric
greenhouse gases and associated temperatures back below potentially dangerous thresholds,
using a state-of-the-art general circulation model. This analysis is extended with a simple
climate model to provide uncertainty bounds. We find that even for very large reductions in
emissions, temperature reduction is likely to occur at a low rate. Policy-makers need to consider
such very long recovery timescales implicit in the Earth system when formulating future
emission pathways that have the potential to ‘overshoot’ particular atmospheric concentrations
of greenhouse gases and, more importantly, related temperature levels that might be considered
dangerous.